Nanoporous hydroxyapatite (Ca 10 (PO4) 6 (OH) 2 ) powders in the form of agglomerates with micrometer dimension were synthesized through hydrothermal wet chemical route. The fine agglomerates were characterized by X-ray diffraction, infrared spectroscopy, scanning electron microscopy and transmission electron microscopy (TEM). Here we use positron annihilation lifetime spectroscopy (PALS) to indicate significant nanoporosity of agglomerates. The longest lifetime component τ 4 , which is ascribed to the annihilation of ortho-positronium (o-Ps) localized at mesopores, provides information about the pore size. τ 4 varied at 300K between 93 and 111ns. The mean size of mesopores D is determined to vary between 6.8 and 11.6nm. The spectra were also analyzed allowing a size distribution.